Method of electromagnetic logging



- Nov. 11, 1941. F. ATHY Em. V2,262,419

METHOD OF ELECTROMAGNETIG LOGGING Filed April 9, 1938A 4 Sheets-Sheet 1 mp//f/ INVENTORS ATTORN EY Nov. 1l, 1941.-

L.. F. ATHY ETAL METHOD 0F ELECTROMAGNETIC LOGGING Filed April 9, 1938 4 Sheets-Sheet 2 ATTORN EY Nov. 11, 1941. L. F. ATHY ET Al. 2,262,419

METHOD OF ELECTROMAGNETIC LOGG'ING Filed April 9, 1938 4 sheets-sheet s Mull/5' s ATTOR Y Nov. l1, 1941. F. ATHY ET AL METHOD OF ELECTROMAGNETIC LOGGING Filed April 9, 1958 4 Sheets-Sheet 4 Patented Nav. 11. 1941 2,262,419 METHOD OF ELECTROMAGNETIC LOGGING Lawrence F.

City, Okla., assgnors to Athy and Harold R. Prescott, Ponca Continental Oil Company, Ponca City, Okla., a corporation of Dela- Ware ' Application April 9, 1938, Serial No. 201,172 9 claims. (c1. mss- 182) Our invention relates to a method of electromagnetic'logging, and more particularly to a method of electromagnetic logging of a bore hole.

In geological explorations, accurate knowledge of the various subsurface strata is obtained by taking cores by means of core drilling. A record or log is kept showing the different formations traversed. This is a tedious operation and sometimes inaccuracies result due to the diiculty of ascertaining exactly when one stratum has been left and another has been entered.

Where a drill hole is already. in existence, it is frequently desirable to check the thicknesses of the various strata. Frequently valuable geological information may be obtained by correlating a plurality of spaced drill holes. For this work Y accurate knowledge of the depth and thicknesses of the various strata is essential.

One object of. our invention is to provide a novel method of electromagnetically logging a bore hole. Anotherobject of our invention is to provide a method of electrical logging of bore holes which will enable the use of small current densities and small potentials while obtaining great accuracy in results.

Still another Aobject of our invention is to provide a method o f electrical logging of bore holes employing a low xed frequency, the apparatus being such that it is non-responsive to fre= quencies higher than the fixed frequency or to frequencies lower than the fixed frequency.

A further object of our invention is to provide Y a method and apparatus for electrical logging of bore holes which will be less susceptible to ground currents and magnetic storms.

A Other an'd further objects of 4our invention will appear from the following description.

In the accompanying drawings which form part vof the instant specification and are to be read in conjunction therewith, and in which like reference numerals are used to indicate like parts in the various views:

amplifier used in carrying out the method of our invention.

Fig. 6 is al schematic view of a rectier unit employed in carrying out our invention.

Fig. 7 is a diagrammatic View showing a pair of record strips over a phantom View -of bore holes in which they were taken.

The electrical logging methods of the prior art are largely based on resistivity measurement. We propose a novel method of electrically logging bore holes in which the magnetic eiects of electrical strata are employed.

Referring now to Fig. 1, an alternator is adapted to deliver alternating potential throughv conductors 3 and 4 to ground stakes l and 2 placed on the opposite sides of a bore hole l300. Current will iiow between current stakes l and 2 in various paths shown by construction lines in Fig. l. A galvanometer 6 is adapted to measure current cwing between current stakes l and 2. Various beds or strata 302, 303 and 304 are shown in Fig. l. Let us assume that bed 30| is of medium resistance, and bed 302 is of low resistance (high conductivity), bed 303 of medium resistance and bed 304 of low resistance, and that the instantaneous current iiow is indicated by the direction of the arrows on the construction lines extending between current stakes I and 2 through the various beds. It will be noted that there are a series of dots and crosses in the bore hole adjacent the constructional lines showing the direction of the instantaneous current flow. A dot indicates that the magnetic flux is flowing at right angles to the plane of the drawing toward the Fig. -1`is a diagrammatic view of apparatus capable of carrying out the method of our inven- I tion.

Fig. 2 is a schematic view showing the exciting source oralternator used in carrying out the method of our invention.

Fig. 3 is a schematic sectional view of a receptor of use in carrying out the method of our invention.

Fig. 4 is a sectional view takenon the line 4 4, Fig. 3.

Fig. 5 is a diagrammatic view of a receiver and observer, that is, a dot will represent the point of an arrow flying toward the observer, while a cross indicates a direction of magnetic flux downwardly from the plane of the drawing, that is, a cross will indicate the feathers of an arrow flying directly away from the observer. The directions of magnetic flux thus indicated can be readily checked by the application of the right hand rule, and 'it will be readily apparent that if the direction of iiux is in one direction in the upper half of the bed it will be in the opposite direction in the lower halt` of the bed. It will also be apparent that changes in direction of magnetic flux will indicate either the passing of the electrical center line of a bed, or the passing from a bed of one conductivity to a bed of another conductivity. A

It is to be understood that the foregoing description is with reference to the diagrammatic Aview shown in Fig. 1,` given to simplify the explanation of our method. In each strata in Fig. 1

the flux direction is indicated for each bed as though the other'strata were not present. The final flux at any position within a stratum is the sum total of the separate flux values contributed by adjacent strata. In actual practice, not only will each bed have a horizontal magnetic flux contributed by its own conductivity and its own magnetic susceptibility, but it will have, too, a magnetic flux which overlaps from the beds on each side. In an actual case, where there are large numbers of geological strata present, each having its individual conductivity and magnetic susceptibility, the prediction of the actual magnitude of the horizontal magnetic flux at any point within the bore hole would be very involved and complex.

It is to be remembered, therefore, that the description given is with respect to the diagrammatic view shown.` The significant thing is that each geological strata has its own value of conductivity and magnetic susceptibility which tendsv to remain constant laterally along the bed. It

.is for this reason that a log of the variations of the magnetic flux in one bore hole may be compared with a similar log of another bore hole to determine the attitude, dip and interval between geological beds.

It will be further apparent to those skilled in the art that only an extreme conductivity between two adjacent strata would cause a net reversal of the eld direction at any point. In the diagrammatic view it is assumed that the adjacent strata are formed of materials possessing extreme differences in conductivity. In actual practice, these conditions are not frequently met except in the case of ore beds or the like.. In the usual case, the variations of conductivity will not be sufficiently great to cause a reversal in the sum total of the flux within the bore hole. 'Ihe differences in conductivity, however, will cause appreciable variations of the total flux, which variations are employed in our invention for logging the bore hole.

For example, in field work a. strata of limestone is frequently found adjacent a strata of shale. The conductivity of shale is often of the order of ten times as great as that of limestone. This difference in conductivity will produce a decided variation in the flux. Again by way of example, the magnetic susceptibility of va strata containing reworked igneous rock frequently is between fifty to a hundred times as great as that of normal sedimentary rock. This difference in magnetic susceptibility will produce a marked variation in the flux.

It will be apparent, therefore, that the flux direction and its magnitude will provide an index of the electrical conductivity of various strata. We provide a receptor susceptible to alternating magnetic flux. This receptor is indicated diagrammatically by the reference numeral 400 and is suspended by a flexible cable 40| housed upon a reel 402, the terminals of the cable being connected to conductors 9, I0, 9 and I0 .by any suitable means, such as brushes 403 and the slip rings. The reel 402 is mounted upon a shaft 404 which is driven through any v suitable transmission by a synchronous motor 405, connected in parallel with another synchronous motor |91, adapted to move a sensitized strip |96 upon whi-ch a record is taken, as will hereinafter be more fully described, power being furnished from main line wires 406 and 401.

The output of receptor 400 is impressed through conductors 9 and |0 upon a receiver and ampliiler indicated diagrammatically by the reference numeral and through conductors 9 and l0' upon a receiver and amplifier indicated diagrammatically by the reference numeral Il. The output of the amplifiers is impressed upon a rectifier indicated by the reference numeral 6 and shown diagrammatically in Figi 6. I'he output of the rectifier is led by conductors l2 and I3 to oscillograph, shown generally in Fig. 1 by the reference numeral I4. may comprise a field magnet |9| and an oscillograph element |92 which is well damped so as to produce low frequencies faithfully in wave form and phase. A mirror |99 is carried by the oscillograph element |92. Light from incandescent lamp |94 is projected by a lens |95 upon the mirror |93 for reflection upon a light sensitive medium |90 provided with an electric motor |91.

for moving the light sensitive medium past the light spot reflected by mirror |93. y,

A resistance |90 is placed in one of the output leads 4 leading to the electrode 2. The resistance is tapped by a variable arm |99. It will be readily apparent that leadsl 200 and 20| across the resistance |90 will indicate the current flowing from the current source 0. The oscillograph element 202 will measure the voltage across the resistance. Since the resistance is fixed, the voltage across the resistance will vary as a function of the current. The oscillograph element 202 is supported within the field of magnet 204, and carries a mirror 200 upon which light from an incandescent lamp 200 is projected by a lens 201. The mirror 200 reflects the light'upon the sensitized fllm |90 in side by side relation with light beams from oscillograph element |92. The oscillograph elements |92 and 202 are quite high in natural frequency, and for the frequencies recorded by them may be regarded as practically without inertia.

A key I0 is adapted to momentarily close the circuit in order to obtain transient effects. By momentarily is meant a short interval of time, as. for example, five seconds.

We employ a low frequency current having a good wave form to energize the transmitter, and we employ a receiver with a high pass filter and va low pass filter, enabling the rejection of currents higher in frequency than the exciting source and currents lower in frequency than the exciting source, followed by the amplification of the filtered current with its subsequent recording upon a record strip or by readings of the current How and voltage employed from which magnetic ux variations may be plotted and recorded to obtain accurate readings in both cased and uncased bore holes.

The alternator shown diagrammatically at 5 is shown in detail in Fig. 2, it being understood that any suitable source known to the art of obtaining a low frequency current of a fixed frequency may be employed in carrying out the method of our invention.

The alternator shown in Fig. 2..is suitable for very low frequencies such as flve cycles per second and at the same time may be employed for frequencies as high as five hundred cycles per second. A vibrating member |9 is carried by a flexible' spring member 20 from any suitable support 2| by means of an adjusting arrangement 22. The natural frequency will be determined by the length and shape of the vibrating member |9 and by the length and stiffness oflthe flexible spring suspension 20. The length of the spring suspension 20 may be varied by adjusting 'I'he oscillograph |4 nutl 23, raising or lowering .the member I8 by means of threaded member 24 as can readily be appreciated by reference to Fig. 2. Coils 25 and nets 10 and 1|, varying the-external magnetic neld and inducingV voltages in the windings 12 and 13, and 14 and 15. The pickup windings 12, 13, 14 and 15 are well separated from the vibrating member I9, are balanced and adjusted to have as nearly as possible a linear relationship between changes' in. the external flux of the small magnets with changes in the position of.

the vibrating member I9. This will give induced electrical voltages substantiallyas free of harmonics as the motion of vibrating member I9.

is adapted to alter the external eld of the small magnets 21 and 30. The voltage induced inthe exciting coils 25 and 26 is led by conductor 3| to one grid 32 of a thermionic tube 33. The voltage induced in coils 28 and 29.is impressed by conductor 34 upon the grid 35 of the thermionic tube 33, the return portion of the circuits being comprised by conductor 36 which is connected to the cathode 31 of the tube 33. The cathode 31 is heated by a larnent heater 38 to which current is supplied from a battery 39.

A battery 40 furnishes bias voltage for the grid. v

A battery v42 supplies positive potential to the plates 43 and 440i the tube 33, through a conductor 45 and center tapped resistance 46, the

return in the plate circuit beingl froml the negative terminal of the battery 42 through conductor.

41 to cathode 31. Plate 43 is coupled by condenser 48 to the grid 49 of thermionic tube 50. Plate 44 is coupled by condenser 5I to the grid 52 of the tube 50. The other side of condenser 48 is connected by conductor 53 to one end of a resistance 54. The other side of condenser 5I is connected to the opposite end of the resistance 54. The resistance 54 is center tapped by a conductor 55 connected to cathode 56 of tube 50. Conductor51 adjustably connects grid.49 to the resistance 54, while conductor 58 adjustablyconnects grid 52 to the resistance 54. The adjust-v ment of conductors 51 and 58 controls the output of tube 50 acting as a volume control. The plate 59 of the tube 50 is connected to the positive terminal of battery 42 through conductor 60, windings 6I and 62, and conductor 63. The plate 64 of the tube 50 is connected to the positive terminal of battery 42 by conductor 65, windings 66' and 61, and conductor 63. The windings 46| and 62 are about a soft iron electromagnet core 68. The windings 66 and 61'are about a soft iron electromagnet core 69. The windings 6|, 62, 66 and 61 are driving windings. The output of tube 50 is controlled to supply suicient energy to keep the member I9 in oscillation. The driving electromagnets 68 and 69 are positioned as far as possible on each side of oscillating memberA |9 and still maintain oscillation. This reduces damping caused by residual magnetism of the cores and allowsmember I9 to oscillate as freely as possible. The amplitude of motion of vibrating member I9 is quite low in order that the motion may be as free of harmonics as possible.A Mounted on each side of oscillating member I9 are small permanent magnets 10 and 1|. Windings 12 and 13 are disposed about the poles of magnet 10. Windings 14 and 15 are disposed about the poles of magnet 1|. As the member I9 of magnetic material 'vibrates, driven by drivingI electromagnets. as described above, it will alternately approach and recede from each of the small permanent mag- The voltages induced in windings 12 and 13 are impressed by conductor 16 upon one grid 11 of thermionic tube 18. The voltages induced in windings 14 and 15 are impressed by conductor 19 upon another grid 80 of the tube 18, the return circuits being completed by conductor 8| to cathode 82 of tube 18. The grids are biased by a C battery 83. The cathode is provided with aflament heater 84 to which current is supplied from an A battery 85. Positive potential from B battery 86 is supplied to the plates 81 and 88 of the tube 18 through conductor 89, center tapped resistance 90 and respective groups of choke coils 9|, 92, and 93, 94 as can readily be seen by reference to Fig. 2. The thermionic tube 18 will amplify the induced voltages genercathode I|1 of tube 96, a biasing C and 14, 15. The amplitubes ated in windings 12,13 fied voltage is passed to thermionic and 96 for further amplication through a low pass filter 91 and a high pass lter 98. The condensers 99 and |00 of the low pass lter are set to .reject frequencies higher than the fundamen-` tal of the vibrating member I9. The condensers |0I and |02 of the high pass filter are adjusted to reject frequencies lower than the fundamental of vibrating member I9. In parallel with choke coil reactances 9| and 92 of the low pass filter are resistances |03 and |04. In parallel with choke coil reactances 93 and 94 are resistances |05 and |06. Resistances are also placed in parallel with each choke coil reactance |01, |08, |09 and ||0 of the high pass filter 98. |01, |08, |09 and I|0 must be quite large in order to provide a peak response at the low frequencies used. The resistances are of such value that the net work is well damped in order that electrical oscillations will not be generated, en-

abling the natural frequency of the vibrating member I9 to be reproduced faithfully in wave form. In this connection, it is unimportant whether or not phase change occurs.

The output of the high pass filter is impressed upon the grids III and I|2 of tubes 95 and 96. the return circuits being completed through common conductor ||3 and conductor ||4 to cathode ||5 of tube 95 and conductor I|6' to battery |I8 being provided. The tubes 95 and 96 are of t-he indirectly heated cathode type and are provided withmlament heaters II9 and |20, respectively, which are suppliedenergy from an A battery I2I. B power is supplied to the plate circuits of tubes 95 and 9,6 by a generator |22. The output of tubes 95 and 96 is connected to the primary winding |23 of the transformerthe power being supplied through conductor |24 to a center tao of the primary |23. Plate |25,of tube 95 is adjustably connected by conductor |26 to taps at one end of the primary |23. Plate |2| of tube 96 is adjustably connected by conductor |28 to taps at the other side of primary |23. The other Side of the generator |22 is connected to cathodes I|9 and |20 by conductor |29,.the cathodes being interconnected by conductor |30.

These inductances The transformer of which winding 23 is a primary must be made with good iron and a high primary inductance in order to efficiently deliver energy at the low frequencies desired and in order to be as free of harmonics as possible. The secondary winding |3I is adapted to conduct the output energy of the alternator to the electrodes I and 2 through conductors 3 and 4. The conductors are connected to the secondary winding I3I oi' the transformer by adjustable connections |32 and |33. It is desirable to have these connections adjustable in order to properly match the impedance of the load circuit to the impedance of the plate circuits of tubes 85 and 96.

For higher frequencies, the spring 20 may be discarded and the vibrating member I may be clamped in the mounting or a tuning fork may be used as the vibrating member. 'When higherv frequencies are used, the harmonics are less and the low pass filter and the high pass filter may be eliminated.

As mentioned hereinbefore, there are other sources of alternating current with" fairly good wave form known to the art which may be used.

The well known beat oscillators using push-pull detection and push-pull amplication can be designed to have a very low harmonic content to frequencies as low'as fifteen or ten cycles per second if the circuits are well separated by buffer stages. Buffer stages can be used in connection with the alternator shown in Fig. 2 where the load delivered by the power stage is great. In this case, a buffer stage will be placed between the thermionic tube 18 and the output tubes 35 and,

The above variations are known to the art and may be used in carrying out the method of our invention. v

The filtered low frequency current flows between ground stakes and 2 through a plurality of paths, the current flowing through various strata varying in accordance with their respective resistivities. In order to determine the flux direction and magnitude we provide a receptor shown diagrammatically in Figs. 3 and 4. It comprises a non-magnetic and if desired a nonmetallic housing or casing 80S in which is positioned a plurality of cores 60|, S32, 603, S04, SI5, 68S. made of a material with a high permeability at fairly low magnetic flux densities.

Our receptor is responsive to horizontal iiux intensity. This i111!!l intensity is determined by the conductivity of layers and also by the magnetic susceptibility of layers. It is well known to the art that the conductivity of different geological beds varies greatly. Apart from their conductivity their magnetic susceptibility may vary depending upon the material of which they happen to be formed. A bed, for example, containing largeamounts of ferrous materials will have a greater magnetic susceptibility than other beds. This fact enables our method to be employed where conductivity methods might fail.

Two beds might have identical conductivities and yet have diii'erent magnetic susceptibilities. Resistance methodsof electrical logging, therefore, would fail to indicate the different beds, which would be discovered when their magnetic susceptibility was taken into account.

` For purposes of illustration we have shown six cores positioned within the receptor by means of lugs 601. The cores are provided with windings 608. The windings of cores 60|, 602 and 603, are connected in series and terminate in conductors 8 and I0, forming elements of the cable 40|. The windings of cores 604, 605 and 606 are connected in series and terminate in conductors 9 and I0', forming elements of the cable By reference to Fig. l, it will be seen that conductors 3 and I0 are connected to amplifier I, while conductors 9' and I0' are connected to amplifier I I.

In practice it is not-practical to orient the receptor, and, therefore, it must be independent of rotational position. In order to achieve this independence We position successive respective cores rotated 30 from each other around the vertical axis. This can be readily seen by reference to Fig. 4. It will be understood, of course, that a slightly less directional receptor may be obtained by the use of a greater even number of cores rotated through a less angle. In this case, however, it will be necessary to increase the length of the receptor. This is undesirable since we prefer to conne the length of the receptor to appreciably less than the thickness of the beds it is desired to explore in order that the receptor itself will not bridge over the magnetic variations sought. The cable 40| passes through the receptor housing cover plate 6I2 through a suitable stuiiing box 6|3. The cover is sealed upon a gasket 6I4. The voltages induced in the coils 608 are, as pointed out above, led by conductors- 9, |0, 9 and I0 for delivery to the receivers and amplifiers, shown diagrammatically by the reference numbers II and II in Fig. 1. Receiver and amplifier II, is shown more in detail in Fig. 5, to which reference is now made. The potential is received through condensers |34 and |35 and impressed at the ends of a resistance |36. The resistance |36 is center tapped by a conductor |31 which is connected to the cathode I 38 of a A thermionic tube |38, the usual C battery |40 being used to bias the grid. The cathode |38 is provided with a filament heater |4| to which energy is supplied by means of an A battery |42. The grids |43 and |44 of the tube |38 are connected to the resistance I 36 by adjustable conductors |45 and |46. The adjustment of these conductors acts as a volume control and varies the potential impressed upon the grids |43 and I 44. Voltage from "B battery |41 is impressed upon plate |48 of tube |33 through conductor |43, reactance |50, reactance ISI, reactance |52 and conductor |53. Voltage from the .B battery |41 is impressedvupon the plate |54 through conductor |48, reactance |55, reactance |56, reactance |51 and conductor |56. 'Ihe output of thermionic tube |33 is impressed upon the grids |53 and |60 oi tubes I6I and |62 through a low pass filter |83 and a high pass nlter |64 through conductors |66 and |66.

The condensers |61 and |68 of the low pass filter are set to reject frequencies higher than the frequency selected to be impressed upon the tubes |6| and |62. The condensers |63 and |10 of the high pass illter are set to reject frequencies lower than the source frequency. It will be noted that each of the reactances in both filters is provided with resistances connected in parallel therewith to provideelectrical damping of the entire network enabling the reproduction of the fundamental frequency of the source faithfully. AIt will be noted that conductors |65 and |66 connecting the output of the high pass filter to the respective grids |53 and |60 may be adjusted upon the resistances I1| and |12 to act as a further means for controlling the overall amplification. By means of the filters, the amplifier delivers a reproduction of the magnetic flux at the V'time earlier.

the receptor up and down the bore hole being receptor with stray effects such as ground currents, electrolytic effects and variable resistance vat the current electrodes materially suppressed.

The grids |59 and |60 return circuits are completed through conductors |13 and respective cathodes |14 and |15, a customary C biasing battery |16 being provided. Filament heater |11 of tube |6| and filament heater of tube |62 are supplied current by A battery |19. Plate |80 r of tube |6| and plate' |8| of tube |62 are connected to the positive terminal B battery |41 through conductor |82 which is connected to a center tap of the primary |83 of the output transformer. The opposite ends of the primary wind-` ing |83 are connected respectively by conductors |84 and |85 to plates |80 and |8| of tubes |6| -and |62, and the plate circuit is completed plified by the amplifier 'I'he lters will reject all useless and extraneous voltages. The

output is impressed upon the anode 100 of a duo diode tube 1 |0, shown in Fig. 6. When the anode 1|0 is positive, current is passed to the cathode 102. This current is ltered by a resistance 103 and a capacity 104 to give a. smoothed out potential which is impressed by conductor 105 upon the grid- 106 of the output tube 101. The current in the output of tube 101 is determined by the potential on the grid 106; The output current of the outputrtube 101 energizes the oscillograph element |92 shown in Fig. 1. Its amplitude is an index of the flux density received by coils Wound on cores 60|, 602 and 603.

Similarly, the voltages induced in coils wound upon cores 604, 605 and 606, are ampliiied by amplifier which is as shown in Fig. 5. The amplified voltage filtered of useless and extraneous voltages is impressed upon the anode `10| of the tube 1|0 in Fig. 6. When this anode is positive, current is passed to the cathode 102. This current is filtered by resistance 103 and capacity 104 and is thus smoothed out and led by conductor 105 to the grid 106 of the output tube 101. The output tube will give the instantaneous summation of all of the voltages induced in the coils 60|, 602, 603, 604, 605 and 606 after being amplified .and filtered.

It can be shown that the construction is such that the receptor is substantially non-directional.

vThe value of resistance 303 and the capacity 304 is such as to smooth out individual humps of the rectified current to give a direct current potential. The time constant, however, is held reasonably small such that the actual value of the smoothed potential is representative of the magnitude of the horizontal flux a short instant of Since the speed of movement of logged is comparatively slow, the small time constant will introduce a time lag comparatively negligible in amount. The amplitude of the 1 oscillograph trace-from the rectifier unit will represent the magnitude of the flux at the posi,-v tion in the bore hole indicated on the recordstrip, which is being moved at a rate which is a function of the rate of travel ofthe receptor along the bore hole.

It will be apparent that a log of the variations` of the magnetic flux made in one bore hole may be compared with a similar log made in another bore hole, enabling a comparison of the two to reveal dip and intervals between geological beds. This is clearly illustrated in Fig. '1. Record strip 800 is spaced according to scale from record strip 80| to indicate the distance between two bore holes. Trace 802 on record-strip 800 is the trace made by oscillograph element 202 of Fig. 1, which shows the frequency of the current source, and

gives its relative amplitude. This can be varied by adjusting resistance arm |99 upon resistance |98. The trace 804 upon record strip corresponds to trace 802 upon record strip 800. Trace 805 upon record strip 80| corresponds to trace 803 upon record strip 800.

Bed 806 was one of low conductivity and low susceptibility. Bed 801 was one of low conductivity and high susceptibility. Bed 808 was one of high conductivity and high susceptibility, while bed 809 was vone of high conductivity and medium susceptibility. Bed 8|0 was one of medium conductivity and low susceptibility. It will be seen that trace 803 is similar to trace 805, enabling the position of the beds in the bore hole in which record strip 80| was made'to be correlated with thebeds in the bore hole in which record strip 800 was made.

The receptor,` if desired, may be constructed with a less direction eiect by the use of more coils uniformly placed about the periphery of the receptor and the use of more than two interconnected coil systems. This, of course, will require additional rectifying tubes and a rectifier v unit with the cathodes all connected to the same point. I

It will be understood that it is normally desirable to make measurements in terms of known quantities. This can be easily accomplished by calibrating the ampliers with the measuring means at the time of eld use. the input leads of the amplifiers to a source of known voltage, the volume control connections |45 and |46, and |1| and |12 may be adjusted to give the desired overall sensitivity.

It will be understood, of course, that the amplifier may be simplied and any suitable amplifier having a low pass filter and a high pass iilter may be employed, in which the grid of the first tube of the amplifier is connected to one lead and the other lead serves as a return to cathode of the rst tube'.

By the use of a filter rejecting all useless frequencies, the amplifying system will be responsive onlyto theV frequency of the source. This allows a much greater amplification to be used,

enabling magnetic variations in the strata around a cased bore hole to be identiiedv and recorded. The casing, of course, tends materially to minimize the magnetic fluxdue to erratic ground currents inside of the bore hole. By the use of an increased amplification made possible By connecting by our iilter arrangement, and the use of a fixed frequency exciting source. the. variations in the magnetic iiux may be deduced from variations in voltage.

In use the receptor 00 is lowered by means of motor 405. This motor is a self synchronous motor and is in parallel/with motor |91 controlling movement of theel/sensitized strip |96. It will be obvious that movement of the record strip will represent movement of the receptor downwardly into the bore hole so that the record will automatically produce curves indicating voltages received at various depths.

It will be observed that at the top of a stratum the voltages received are at a maximum and that at a middle point of the stratum they are at al minimum. 'I'here is a change, too, from maximum to minimum when one stratum is left and another entered. The resistivities and magnetic susceptibilities of the various strata may be determined by the amplitude of the induced voltages. When the conductivity of a stratum is high the induced voltages are higher than in the case when the resistivity of a stratum is high. A porous stratum or one bearing oil or gas is usually of high resistivity and the induced voltages received will usually be of low amplitude. The record strips may be placed with the inter-strip distance' proportionate to the inter-bore hole spacing and a profile of the geological section may be drawn as indicated in Fig. 7. In this manner faults and other discontinuities in strata may be-readily determined.

If desired, the motion of the record strip may be increased. Velocity and transient effects may be observed by momentarily closing the key I6 in Fig. 1. In such case the record strip need not be moved in proportion to the position of the receptor, and the resultant record strip may be later cut up and placed on a plotting sheet at positions proportionate to the depths at which the strata were investigated. The use of transient effects gives an additional eiIect since various strata possess in addition t9 different resistivities and magnetic susceptibilities, other characteristics which will make for a more rapid orless rapid damping of an electrical impulse passing therethrough.

The condition of the fluid in the bore hole has practically no contributinginiluence on the results of the electro-magnetic survey. The receptoris responsive only Kto horizontal ilux variations and any vertical currents in the hole will have little disturbing eifect.

It will be observed that we have accomplished the objects of our invention. .We are able to produce bore hole logs by electromagnetic methods withl great accuracy and can minimize the masking effects of `electrical storms, eiectrolytic effects of various strata, polarizing effects and other stray electrical effects. f'

We are enabled to employ smaller equipment rendering it more easily portable. 1

` It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of 'our claims. It is further obvious that various changes may be made in details within the scope of our claims without de'- parting from the 'spirit of our invention. It'is, therefore,` to be understood that our invention is not to be limited tothe speclc details shown and described.

Having thus described our invention, what we claim is:

1. A method of electromagnetically logging bore holes in the earth including the steps of passingan alternating current of predetermined frequency between two points disposed on opposite sides of the bore hole, whereby said current will flow through various subsurface strata at different densities in accordance -with respective conductivities ot the strata, thereby inducing alternating magnetic fields in the bore hole adjacent various strata, said magnetic elds varying in intensity as a function of the respective conductivities of the strata, and measuring the intensity and instantaneous direction of the induced magneticileld adjacent successive strata.

2. -A method of electromagnetically logging bore holes in the earth including the steps of passing an alternating current of predetermined frequency between two points disposed on opposite sides of the bore hole, whereby said current will iiow through various subsurface strata at diiferent densities in accordance with respective conductivities of the strata, thereby inducing alternating magnetic fields in the bore hole adjacent various strata, said magnetic fields varying in intensity as a function of the respective conductivities of the strata, and measuring the horizontal component of the magnetic field adjacent successive strata.

3. A method of electromagnetically logging a bore hole in the earth including the steps of passing an alternating current of predetermined frequency through the earth between two separated points dLsposed on opposite sides of a bore hole whereby current will now through various subsurface strata at different densities in acoordance with respective conductivities of the strata, generating induced alternating voltages from the induced magnetic fields of various strata at points along the bore hole, and measuring the amplitude of said induced voltages.

4. A method of electromagnetically logging a bore hole in the earth including the steps of passing an alternating current oi' predetermined frequency through the earth between two separated points disposed on opposite sides of a bore hole whereby current will flow through varios subsurfacestrata at different densities in Iducing current, and measuring said induced volt- 5. A method of electromagnetically logging a bore hole in the earth including the steps oi' passing an alternating current of predetermined frequency through the earth between two separated points disposed on opposite sides of a bore hole whereby current will flow through various subsurface strata at different densities in accordance with respective conductivities of the strata, generating induced alternating voltages from the induced magnetic fields of various strata at points along the bore hole, rejecting voltages lower in frequency than the' frequency of said inducing current, and measuring said induced voltages. l

6. A method of electromagnetically logging a bore hole in the earth includingthe steps of passing an alternating current of predetermined ingfan alternating current through the earth p frequency through the earth between two separated points disposed on opposite sides of a bore hole whereby current will ow through various subsurface strata at different densities in accordance with respective conductivities of the stratafgenerating induced alternating voltages from the induced magnetic fields of various strata at points along the bore hole, rejecting voltages higher in frequency than the frequency of said inducing current, and measuring said induced voltages.

'7. A method of electromagnetically logging .a bore hole in the earth including the steps of passing an alternating current of predetermined frequency through the earth between two separated points disposed on opposite sides of a bore hole whereby current will ilow through various subsurface strata at different densities in accordance with respective conductivities of the strata, generating induced alternating voltages from the induced. magnetic fields of various strata at points along. the bore hole, and measuring said induced voltages.

8. A method of electromagnetically logging a bore hole inthe earth including the steps of passbetween two separated points disposed on opposite sides'oithe bore hole whereby current will Vflow through the various subsurface strata at different densities in accordance with the respective conductivities of the strata thereby inj ducing magnetic elds in accordance with the,

current flow therethrough, rand measuring the intensity and direction of the induced magnetic fields along the bore hole.

9. A method of electromagnetically logging a bore hole in the earth including the steps of passing a current through the earth between two separated points disposed on opposite sides of the bore Ahole,V whereby current will ilow through various subsurface strata. at diierent densities in accordance with the conductivity of respective strata to produce induced magnetic elds, moving a conductor along the bore hole, and measuring the ilux density of the induced magnetic elds adjacent various strata as a function of the current flowing in said conductor.

LAWRENCE F. ATHY. HAROLD R. PRESCO'IT. 

